And it's not just hybrids; there are complaints about the real-world mileage of Ford's various EcoBoost turbocharged engines as well.

So how did the C-Max mileage mess happen--and what does it mean for buyers, for the EPA gas-mileage rating system, and for Ford itself?

C-Max Hybrid not tested

Ford said during the call that it had not actually tested the gas mileage of the C-Max Hybrid, but simply used the ratings from the 2013 Fusion Hybrid mid-size sedan instead.

An EPA rule permits vehicles that use the same powertrain and are in the same weight class to use the ratings from tests of any single vehicle in the group without separate tests.

The rule reduces testing costs for automakers. Ford said it customarily tested the higher-volume vehicle among such similar models.

(One example from the early days of EPA regulation: The last MG Midget sports cars from 1976 to 1979 used the engine and transmission from the Triumph Spitfire sports car also made by the same company, meaning the lighter Midget didn't need to be separately tested.)

Surprise and questions

But few outside Ford realized that Ford had not tested the new C-Max separately; the revelation sparked a number of questions from reporters during the media teleconference.

The company is "now starting to understand hybrid variability" and "anomalies," Ford said on its conference call.

It said it has worked with the EPA, which has been investigating C-Max gas mileage, and concluded that "hybrids have a lot more variability and sensitivity to variables."

Accordingly, the company has now begun testing its C-Max models separately.

Same procedure for plug-ins

Ford noted that it had used the same approach with its C-Max Energi and Fusion Energi plug-in hybrids.

In that case, it tested the C-Max plug-in--which it expects to be the higher-volume vehicle of the pair--and used that 43-mpg rating for the Fusion Energi as well.

Had it tested the plug-in Fusion separately, Ford suggested, that car may have achieved a slightly higher rating on its own.

Industry analyst Dave Sullivan of Autopacific later asked Ford whether it had done the same for non-hybrid vehicles that are in the same weight class and use identical powertrains.

EPA statement: few clues

Many commentators have questioned whether the EPA will revisit the way it measures and certifies fuel economy for hybrid models, in light of Ford's troubles with the C-Max.

One option: The agency could revise the "adjustment factors" for hybrid vehicles.

It's already done that once, in 2007, when it re-rated a number of the earliest hybrids--including the first-generation Honda Insight, the Honda Civic Hybrid, and the Toyota Prius--to compensate for overly high ratings that weren't achieved by owners in real-world use.

A three-page EPA statement on the C-Max rating change offers few clues to what may happen in future.

It says that it expects carmakers to use high-efficiency powertrains across more vehicles in the future, and that it welcomes this trend.

The agency said it will "be working with consumer advocates, environmental organizations, and auto manufacturers to propose revised fuel economy labeling regulations to ensure that consumers are consistently given the accurate fuel economy information on which they have come to rely."

Not all MPG differences are the same

The statement also reiterates that the relabeling was "voluntary"--a point very heavily stressed by Ford--and notes that the ratings apply to all 2013 C-Max Hybrids, even including those that have been updated with new software.

It notes that the EPA's own tests showed the C-Max got "significantly lower" mileage than its rating, owing to "aerodynamic characteristics" that produced a "significant difference in fuel economy [than] the Fusion Hybrid."

But the interesting part of the EPA's statement is this:

This is a new and emerging issue caused by a combination of factors. Because high fuel economy vehicles use so little fuel per mile, relatively small changes in fuel consumption can have a surprisingly large impact on miles per gallon. Advanced manufacturing techniques are making it much easier for automakers to produce vehicles with the same engine and transmission, but with a wider variety of other design changes. The fuel economy of advanced technology vehicles, which involve sophisticated designs and control mechanisms, can be particularly sensitive to small design changes. All of these factors came together in 2013, which was the first year when manufacturers began to market families of hybrid vehicles.

To date, most high-efficiency hybrids have been used in a single vehicle, ensuring a unique and accurate label for those advanced vehicles. The Ford hybrid family is one of only two example in the industry where advanced technology vehicles with the same engine, transmission and hybrid components are used across multiple vehicle designs. EPA regulations allow but do not require automakers to generate a label for each design in this circumstance.

With the new Ford C-Max label, each vehicle design within the two high-efficiency hybrid families now has its own label.

Conventional vehicles often share the same engine and transmission across multiple products, but are far less sensitive to the variations among these products and hence have nearly identical fuel efficiency.

Consumers overly upset?

This is a point that shouldn't be understated. The difference in both fuel consumed and dollars at the pump between 43 mpg and 47 mpg is minuscule compared to an "equivalent" 4-mpg difference between, say, 10 mpg and 14 mpg.

Here's the math: Over 15,000 miles, the reduction from 47 to 43 mpg will use an extra 30 gallons of fuel. At $4 a gallon, that's an extra cost to the owner of $120 a year.

The reduction from 14 to 10 mpg uses an extra 429 gallons, at a cost of $1,716.

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